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A fleet of 12 Toyota Priuses in London have been fitted with GPS-linked speed limiters to measure how drivers respond to having their speed controlled by a computer rather than their own feet.

For now, the trial is local to London, run by Transport for London, the agency that manages the city's roads, buses, subways and trains. The agency is using its own fleet of Priuses for the test and will add a bus and a taxi later this year.

The experiment, which Transport for London also says will help determine whether accident rates can be reduced by the technology, uses a black box that combines GPS location data with a speed database and overrides the electronic throttle if the driver pushes past the posted limit.

It would seem laudable, except that some see sinister motives at work. The test is linked to road tolls, and the idea has a strong resonance with control-freak politicians and bureaucrats, who like the idea of charging and controlling Europe's motorists at every opportunity. Speed limiters linked to digital maps of speed limits raise the specter of speed control by decree of the European Commission. Big Brother is, without a doubt, lurking in Europe's collective mind.

The trial has being going for just more than a month, and so far, the evidence is generally anecdotal. "Most drivers didn't like the idea at first," said Chris Lines, Transport for London road safety chief. "But some start to like it. It saves them getting speeding tickets."

Downsides? "The main thing they notice is the big queue of vehicles that builds up behind them sometimes," Lines noted.

Drivers in the trial get a switch to choose how the speed limiter operates. Advisory mode gives a dashboard warning when the speed limit changes. Voluntary mode overrides the throttle so it slows the car and prevents the driver from speeding.

One potential problem is that GPS signals fade in the shadow of tall buildings in big cities. Lines admits that this does happen, but the system is programmed to assume that the limit is unchanged until the signal reconnects.

Like many road-safety campaigners, Lines is very persuasive with his argument. "There are 200 people killed and 2,500 seriously injured on London's roads each year, and it costs £1.2 billion [about $1.9 billion]," he said.

But the jury is still out on whether taking control of a car's accelerator is the right way to solve that problem.